Add posthog-rails gem for automatic Rails exception tracking

This commit introduces the posthog-rails gem which provides automatic
exception tracking for Rails applications, including:

- Automatic capture of unhandled exceptions via Rails middleware
- Automatic capture of rescued exceptions (configurable)
- Automatic instrumentation of ActiveJob failures
- Integration with Rails 7.0+ error reporter
- Configurable exception exclusion list
- User context capture from controllers

Core library improvements:
- Added comprehensive logging throughout exception capture flow
- Added ExceptionCapture module for standardized exception parsing
- Fixed field handling in Transport to strip internal-only fields
  (type, library, library_version, messageId) that are not part of
  PostHog's RawEvent struct
- Fixed UUID field handling to avoid sending null values
- Added capture_exception method to Client for explicit exception tracking

The posthog-rails gem includes:
- Railtie for automatic initialization and middleware insertion
- Multiple middleware layers for capturing different exception types
- ErrorSubscriber for Rails 7.0+ error reporter integration
- ActiveJob extensions for job failure tracking
- Comprehensive configuration options

Author: popcorn

CHANGELOG.md

@@ -1,3 +1,13 @@
+## Unreleased
+
+1. feat: Add posthog-rails gem for automatic Rails exception tracking
+   - Automatic capture of unhandled exceptions via Rails middleware
+   - Automatic capture of rescued exceptions (configurable)
+   - Automatic instrumentation of ActiveJob failures
+   - Integration with Rails 7.0+ error reporter
+   - Configurable exception exclusion list
+   - User context capture from controllers
+
 ## 3.3.3 - 2025-10-22
 
 1. fix: fallback to API for multi-condition flags with static cohorts ([#80](https://github.com/PostHog/posthog-ruby/pull/80))

README.md

@@ -11,6 +11,10 @@ Specifically, the [Ruby integration](https://posthog.com/docs/integrations/ruby-
 >
 > All 2.x versions of the PostHog Ruby library are compatible with Ruby 2.0 and above if you need Ruby 2.0 support.
 
+## Rails Integration
+
+**Using Rails?** Check out [posthog-rails](posthog-rails/README.md) for automatic exception tracking, ActiveJob instrumentation, and Rails-specific features.
+
 ## Developing Locally
 
 1. Install `asdf` to manage your Ruby version: `brew install asdf`

lib/posthog/logging.rb

@@ -41,8 +41,8 @@ def logger
         return @logger if @logger
 
         base_logger =
-          if defined?(Rails)
-            Rails.logger
+          if defined?(::Rails)
+            ::Rails.logger
           else
             logger = Logger.new $stdout
             logger.progname = 'PostHog'